2 92 Dr. Brewster on the multiplication of images , &c. 
till the colours of the two apertures are equally intense, we 
obtain the angles of incidence at which steel and silver pola- 
rise equal quantities of light in the plane of reflection. By 
using two plates of black glass having a variable inclination 
to the axis of the tube, we may allow the light to fall at equal 
angles upon the two metals, and thus ascertain the different 
inclinations of the plates of glass at which the two apertures 
exhibit the same intensity of colour. 
I have the honour to be, &c. 
DAVID BREWSTER. 
Edinburgh, May I, 1815. 
To the Right Hon. Sir Joseph Banks, Bart. G. C. B. P. R. S. &c. &c. &c. 
